OAuth refresh token error - iphone

Question

I'm using OAuth 2.0 to sign into a website. so each time when i try to log into the server i got a response that refer to an expiration date, access token and refresh token. the problem is the token is expired before the given time that i got from the server. so i figure out that there is an interval between the time of the server and the time of the iPhone. when i looked at the code of the SDK facebook there is no logic to handle this issue, it's a simple comparison. so my question is this issue from server side i mean the implementation of OAuth is incorrect or it's an issue from the client side?
Facebook Code:
- (BOOL)isSessionValid {
return (self.accessToken != nil && self.expirationDate != nil
&& NSOrderedDescending == [self.expirationDate compare:[NSDate date]]);
}
My code :
// Get the remaining period of the token to expire and subtract 30 sec
int delay = ([expirationDate timeIntervalSinceDate:serverDateTaken] - 30);
// Save the new Expiring date
objectOAuth.expiresIn = [[[NSDate date] dateByAddingTimeInterval:delay] description];
+ (BOOL)isSessionValid
{
// Get expiration date
OAuth *authParam = [Connection getSessionParameters];
// Formating
static NSString *timeZone = #"UTC";
NSDate *expirationDate = [Connection getDateFromString:authParam.expiresIn withTimeZone:timeZone];
// get the remaining period
int diff = [expirationDate timeIntervalSinceNow];
// Check if it's expired
return (diff <= 0);
}

Answer 1

Unfortunately there isn't really any good mechanism to check the time skew between the client and the server. I would stick with the simple check since there is less code that needs to be maintained and debugged. And no matter how much you check, you will still need to handle getting back a TOKEN EXPIRED error.